Alkylene oxides, such as ethylene oxide and propylene oxide, are some of the most important mineral oil-based industrial chemicals. Ethylene oxide (EO) in particular is a starting material for the preparation of ethylene glycol, which is added, for example, to aviation gasoline as an antifreeze. Since ethylene oxide and propylene oxide are additionally also depleted by a reaction with all kinds of substances which possess acidic hydrogen atoms, they are suitable, for example, for addition onto alcohols or amines to form polyalkylene glycol chains which impart a hydrophilic character to these substances. The significant outlet for this type of compounds is that of nonionic surfactants, which find use especially in washing compositions and cosmetics. The reaction of ethylene oxide or propylene oxide with ammonia affords alkanolamides; with carbon dioxide, ethylene carbonate and propylene carbonate are obtained, which are likewise intermediates of interest for industrial chemistry.
Ethylene oxide and propylene oxide are now prepared nearly exclusively by direct oxidation of the corresponding alkylenes over silver catalysts:

The reaction is, for example, exothermic by 120 kJ/mol for ethylene oxide and competes with the complete combustion of the ethylene to carbon dioxide, which proceeds significantly more exothermically at more than 1300 kJ/mol. Ethylene oxide is prepared industrially, for example, generally in tube bundle reactors which may contain up to 1000 individual tubes and are cooled from the outside with a liquid heat carrier, for example tetralin, in order also to be able to maintain the oxidation temperature of from 230 to 270° C. even in the case of increasing total oxidation. The catalyst, for example 15% by weight of silver on Al2O3, is present as a bed in the tube. In general, preference is given to oxidation with oxygen. Nevertheless, the conversion of ethylene is limited to about 10 to 15%, since only in this way can selectivities of not more than 75 to 80% be achieved. About one quarter of the expensive starting material is thus combusted to carbon dioxide in this way. An additional factor is that typically up to 2.5% by volume of water and up to 10% by volume of carbon dioxide are present in the end product, which have to be removed before the further utilization with a high level of technical complexity. The problems in the preparation of propylene oxide are comparable.
An article by Schüth et al. in Ind. Eng. Chem. Res 41, 701-719 (2002) discloses the use of microreactors for silver-catalyzed oxidation of ethylene to ethylene oxide. The oxidizing agent used here is pure oxygen. German patent DE 10257239 B3 (ACA) discloses a process in which olefins, for example including ethylene, are oxidized in the presence of a photosensitizer, the reaction being effected in a multitude of micro-falling-film reactors running parallel to one another. The oxidizing agent used here too is oxygen. International patent application WO 01/083466 A1 (Merck) proposes a process for epoxidation of functionalized olefins, in which the reaction is likewise effected in microreactors, but under mild conditions in the liquid phase. The reaction is additionally performed without catalysts. A similar process for preparing active pharmaceutical ingredients is known from Japanese patent application JP 2004-285001 A2 (Sumitomo): here, it is proposed to react on saturated feedstocks, for example methyl 3-dimethyl-2-(2-methyl-1-propenyl) cyclopropanecarboxylate, with ozone in the liquid phase, and the reaction can take place in a microreactor; but the oxidation of ethylene or propylene is not mentioned here.
It was therefore an object of the present invention to provide a process for industrially preparing ethylene oxide or propylene oxide, which is free of the disadvantages of the prior art and, especially at high conversions, affords improved selectivities and space-time yields, and suppresses the undesired total oxidation of the feedstocks as far as possible.